Currently used packaging materials for thin film wiring structures fail to meet three major requirements:
(1) Adhesion (glass ceramic and self-adhesion to fully cured polymer) PA0 (2) Low stress (low thermal expansion coefficient (TEC) or relaxation mechanism) and PA0 (3) Low swell when in contact with N-methylpyrollidone (NMP).
The poor adhesion observed for many polyimides is often circumvented with adhesion promoters and reactive ion etching (RIE) surfact treatments, but the reliability of this approach is suspect in temperature and humidity testing and can lead to corrosion. A film which is low stress and does not swell is desired to avoid cracking, delamination, etc. of the polymer films during fabrication. Biphenyl dianhydride-phenylene diamine (BPDA-PDA) is the best commercially available polyamic acid to meet these requirements, but the adhesion characteristics of this material are poor. Alternatively, Poly(phenylquinoxalines), PPQ, show excellent adhesion characteristics to a wide variety of substrates but these materials cannot be processed from N-methyl-pyrollidone (NMP) and have a high TEC.
Ideally a hybrid material (copolymer) can be developed which displays excellent adhesion and low NMP swell like PPQ's, the processing characteristics of polyamic esters, and the final properties of rigid or semi-rigid polyimides after cure. There are a number of reports concerning polyimide-phenylquinoxaline based random copolymers, but these materials must be processed from m-cresol. There are no reports of polyimidephenylquinoxaline block copolymers primarily due to the lack of a common solvent system for polyamic acids and PPQs to allow copolymerization, and the difficulty encountered in performing block copolymerizations with polyamic acids of rigid and semi-rigid polyimides.
Chemical Abstracts 109:55338r shows the synthesis of polyphenylquinoxalines via aromatic nucleophilic displacement, but it is not concerned with the block copolymers of the present invention.